Gear-type pumps and motors typically employ a housing of cast iron or the like into which is formed a pair of pockets. The pockets are arranged in a slightly overlapping relationship so that when the gears are installed therein, the gear teeth enmesh. Each of the gears is supported on a journalled shaft for rotation. The pockets and the gear teeth are cooperatively formed to provide virtually zero clearance to slight clearance between the ends of the teeth and the pocket wall. The gears and their shafts are confined by the housing which supports one end of the gear shafts. A housing cover is used to support the other end of the gear shafts. Used with appropriate seals, the housing and housing cover retain all of the interior parts and prevent fluid leakage to the machine exterior.
In the simplest type of gear machine, there is a slight dimensional difference between the thickness of the gears and the depth of the pockets, the pockets being slightly deeper. This dimensional difference is necessary to permit the gears to freely rotate. However, gear machines which are so designed tend to exhibit unacceptably high leakage characteristics. For example and assuming such a gear machine is used as a pump, fluid from a reservoir will be drawn by the rotating gears into the inlet port to fill the spaces between the teeth and the pocket wall. As the gear teeth approach and come to meshing engagement, fluid is expelled to the discharge port at an elevated pressure.
Because of this pressure differential between the discharge port and the inlet port, "cross face leakage" will occur. That is, fluid under pressure will leak from the discharge side across the faces of the gears to the inlet side of the pump. This leakage not only significantly impairs the volumetric efficiency of the pump but it also contributes to excessive heat within the housing. Gear motors which receive high pressure fluid at the inlet port and expel fluid at a low pressure from the discharge port also suffer from these same leakage-related deficiencies.
A known approach toward solving the problems described above is to provide a seal arrangement which uses a thrust plate located between one set of gear faces and the housing cover. Some seal arrangements of this type are configured for machines adapted for only a single direction of rotation. Examples of such arrangements are shown in U.S. Pat. Nos. 3,104,616 and 3,904,333. Other arrangements such as those shown in U.S. Pat. Nos. 3,961,872; 4,292,013 and 4,830,592 are contemplated for use in machines having bi-directional capabilities.
When equipped with certain types of sealing arrangements involving thrust plates as described above, the performance of gear motors also tends to become impaired in ways other than those involving cross face leakage. Specifically, thrust plates may be urged toward the adjacent gear faces with excessive force if the seal arrangement is improperly designed. This tends to squeeze out the thin film of oil or other fluid which would otherwise form between the plate and the gear face. The result is that friction increases and the motor exhibits poor torsional characteristics when starting under load or when decelerating to a stop. On the other hand, if the thrust plate is urged toward the gear faces with too little force, torsional efficiency will improve but cross face leakage may increase to an intolerable level.
The arrangement shown in U.S. Pat. No. 4,830,592 is a refinement of earlier efforts and uses grooves formed in the thrust plate to receive seal assemblies. Another refinement is offered by the arrangement of U.S. Pat. No. 4,292,013 which uses lands and seal assemblies in an effort to resolve the foregoing problems.
It is recognized by designers of gear machines that there is a tendency toward severe internal leakage and a need for just the right amount of sealing force acting on a thrust plate in those localized portions of the machine which are at higher pressure. On the other hand, these design parameters tend to decrease in criticality with decreases in localized pressures. It is also recognized by such designers that thrust plates commonly used in gear machines can, by proper seal arrangement, be made to bend or warp slightly. The result of such an arrangement is that for any given localized pressure zone, the sealing force between the surface of the thrust plate and the face(s) of the gear(s) is generally proportional to the localized pressure. As a result, steps have been taken to configure sealing arrangements in such a way that selective areas of a thrust plate are urged toward a gear face in a way to reduce localized internal leakage to an acceptable level while yet avoiding undue friction.
Efforts to achieve the foregoing have involved forming grooves in the thrust plate and employing sealing arrangements therein which function to divide the grooves into several chambers. These chambers are substantially pressure isolated from one another and selectively confine pressurized fluid for each direction of rotation of the machine. Since these grooves and seal assemblies are used on that side of the thrust plate which is away from the gear faces, the resulting chamber pressurization will tend to urge the thrust plate toward the gear faces in a localized way.
The prior work in this field tends to be characterized by certain disadvantages. As mentioned above, some of the arrangements described in the foregoing patents tend to be unsuitable for bi-directional machines. Therefore, customer needs for machines having either one of two directional capabilities can be met only by making, stocking and using separate sets of parts. This unnecessarily complicates the manufacturing process. In addition, it requires that distributors of such products maintain stocks of pumps and motors which are suitable for each of both directions of rotation. The higher levels of required stock have an adverse effect upon inventory costs and, ultimately, upon business profits.
A sealing arrangement for a gear machine which uses one or two thrust plates having easily formed grooves, which employs seal assemblies that are relatively simple in construction and which results in a gear machine with superior torque and leakage characteristics would be a distinct advance in the art.